1. Field of the Invention
The present invention relates to a circuit board for high-capacity modules. More particularly, the present invention relates to a circuit board used in high-capacity modules including power modules, such as an inverter used in electric vehicles (for example, a hybrid car (HV), an electric vehicle (EV), etc.) and various electric appliances, etc. Moreover, the present invention relates also to a production method of the circuit board.
2. Description of Related Art
Recently, in electric vehicles (for example, a hybrid car (HV), an electric vehicle (EV), etc.) and various electric appliances, etc., high-capacity (large electric power) modules including power modules, such as an inverter, are used increasingly widely. Generally, such a high-capacity (large electric power) module includes, for example, a circuit (may be referred to as a “power circuit” henceforth) containing a power semiconductor element, such as a switching element (for example, IGBT (Insulated Gate Bipolar Transistor), etc.) and, for example, a peripheral circuit (may be referred to as a “drive circuit” henceforth) which controls such a power semiconductor element.
In addition, in the present specification, a high-capacity module refers to a module handling a voltage of 200V or more, or a large electric power of 10 A or more. As a specific example of such a high-capacity module, for example, what is called a “power module” etc. can be exemplified.
In a high-capacity module as mentioned above, reduction of a noise which arises, for example from a power semiconductor element, such as a switching element, has been an important technical subject. Specifically, for example, an abnormality may arise in switching operation of a switching element due to a noise which arises as a result of switching operation of the switching element and lead to destruction of a power circuit containing a power semiconductor element, such as a switching element, or a peripheral circuit. Furthermore, such a noise may be revealed to the exterior of a high-capacity module to affect operation of a peripheral equipment of the high-capacity module.
Additionally, in the art, as a remedy for loss in a power semiconductor element including, for example, IGBT, MOSFET, etc., technology trends, which use a silicon carbide (SiC) wafer or a gallium nitride (GaN) wafer in place of a silicon (Si) wafer used conventionally, are becoming remarkable (for example, SiC-IGBT, SiC-MOSFET, GaN-IGBT, GaN-MOSFET, etc.). In a semiconductor element which uses such a new type of wafer, since operation in higher switching frequency is attained as compared with a semiconductor element which uses a conventional Si wafer, there is an advantage that miniaturization of a high-capacity module is attained. However, since the frequency of a noise which arises from these semiconductor elements also rises with the rise of switching frequency, problems resulting from a noise as mentioned above also become more serious. Therefore, in a high-capacity module, reduction of a noise which arises from a power semiconductor element has been an increasingly important technical subject.
It is known that it is effective as a countermeasure for a noise as mentioned above to connect a capacitor (what is called a “snubber capacitor”) in parallel with a power semiconductor. A snubber capacitor has an effect which suppresses voltage change accompanying switching operation of a power semiconductor element. In order to reduce a noise more effectively by such a snubber capacitor, it is necessary to shorten distance between a power semiconductor element and a snubber capacitor. It is because the longer a wiring (wire) which electrically connects a power semiconductor element and a snubber capacitor becomes, the larger an equivalent inductance of the wiring becomes, and thereby surge voltage induced due to a noise which arises with switching operation increases, and as a result a noise reduction effect by the snubber capacitor is not sufficiently demonstrated.
However, in a conventional high-capacity module, as shown, for example, in FIG. 1, since it is necessary to attach an external snubber capacitor 126 to the exterior of a high-capacity module 100, the wiring (wire) which electrically connects a power semiconductor element 113 and the snubber capacitor 126 becomes long, and could not sufficiently demonstrate a noise reduction effect by the snubber capacitor 126. In addition, in a conventional high-capacity module, a power circuit containing the power semiconductor element 113 and a peripheral circuit which contains the control circuit element 125 which controls the power semiconductor element 113, for example, are arranged planarly, and an area for arranging a wiring (wire) 116 for connecting these circuits is required. These have been factors in preventing a high-capacity module 100 from reduction in size and weight. Moreover, problems such as long wiring length due to wire distribution which connects the various circuits which constitute the high-capacity modules 100 as mentioned above and large loss as a whole module, have been also recognized.
Then, it has been proposed to attempt to laminate substrates of various circuits which constitute a high-capacity module as mentioned above to attain reduction in size and weight of the high-capacity module as well as improve a connection form between the various circuit boards which constitute the high-capacity module to attain reduction in loss of the high-capacity module (refer to PTL 1 to 3). Moreover, in the art, as shown, for example, in FIG. 2, in a high-capacity module 100 which has such a laminated structure, a configuration where a snubber capacitor 126 is mounted on a circuit board 121 of a peripheral circuit (drive circuit) 120 which controls a power semiconductor element 113 has been proposed. Although such a configuration can shorten a wiring (wire) which electrically connects the power semiconductor element 113 and the snubber capacitor 126, as compared with a configuration where a snubber capacitor 126 is disposed in the exterior of a high-capacity module 100 as previously mentioned, its effect is restrictive and further reduction in surge has been demanded.
Then, in the art, as shown, for example, in FIG. 3, in a high-capacity module 100 which has a laminated structure as mentioned above, a configuration where a snubber capacitor 126 is embedded inside a peripheral circuit (drive circuit) board 121 has been also proposed. According to such a configuration, wiring which electrically connects a power semiconductor element 113 and the snubber capacitor 126 can be further shortened.
On the other hand, for example, in a circuit board which uses ceramics as a substrate, when an inner layer electrode, which is embedded inside a circuit board, and a substrate are simultaneously sintered, since a ceramics material which constitutes a substrate has a behavior in densification (contraction profile) different from that of a conductor (for example, metal) which constitutes an inner layer electrode, a stress resulting from difference of amount of contraction between the substrate and electrode acts. Especially, in a circuit board assumed to be used in a high-capacity module, although it is desirable to enlarge thickness of an inner layer electrode in order to reduce loss as a whole module, the larger the thickness of the inner layer electrode becomes, the larger the stress resulting from a difference in the amount of contraction between the substrate and the electrode becomes, and therefore a possibility for a problem, such as a crack arises in the substrate due to the stress, to occur increases. When a crack arises in a substrate, moisture may enter through the crack to lead to corrosion of an inner layer electrode and decrease in insulation property, and may further lead to disconnection of the inner layer electrode.
As the above, in the art, there has been a demand for a circuit board including a substrate consisting of dielectric layer(s) comprising mainly ceramics for a high-capacity module, wherein an inner layer electrode embedded inside the circuit board and the substrate are simultaneously sinterable without an occurrence of a crack in the substrate.